Platelets are one kind of blood cells having a diameter of 2 to 4 μm, and play a key role in hemostasis and thrombosis in a living body. It is known that platelets are produced as a result of differentiation of undifferentiated hematopoietic cells in the bone marrow into megakaryocytes through megakaryocyte precursor cells, followed by fragmentation of the cytoplasm of the matured megakaryocytes.
Platelet transfusion has been widely used for cancer chemotherapy and treatment of blood diseases such as thrombocytopenia. The current platelet transfusion that relies upon volunteer donors has problems such as
1) a frequent recipient gradually suffers from a platelet transfusion refractory state due to platelet alloantibodies (derived from HLA or HPA),
2) there is a virus infection risk due to transfusion in the window period, and
3) it takes time to collect platelet components and this places a burden on donors, and in medical front platelet products have been deficient due to a decrease in the number of donors year after year. A short storage life (three days) is also one of the reasons for a deficiency in platelet products. Therefore, a method that stores platelets at a low temperature for a long time by modifying the surface sugar chains of platelets has been proposed (Non-patent Document 1). However, this method has not been put to practical use.
Administration of thrombopoietin (TPO) to a patient has been examined as cytokine therapy for thrombocytopenia due to cancer chemotherapy. However, the expected effect has not been achieved. Contrary, the problem of occurrence of an anti-TPO antibody was revealed, and the development has been stopped. Artificial platelets have been developed (Non-patent Document 2), but have not been put to clinical application yet.
In view of the above situation, transfusion alternative therapy (i.e., in vitro platelet production technology) that produces a large amount of platelets by culturing (inducing differentiation of) undifferentiated hematopoietic cells (mainly hemopoietic stem/precursor cells) in vitro, and returns the produced platelets to the living body has been extensively studied along with development of regenerative medicine technology. If platelets can be mass-produced due to development of in vitro platelet production technology in the future, the current blood donation system will become unnecessary, and a deficiency in platelet products and a virus infection risk will be eliminated.
Hematopoietic stem cells can be mainly collected from the bone marrow, cord blood, and peripheral blood as a cell source. When using such a cell source, platelet-transfusion refractoriness and a virus infection risk can be completely eliminated by inducing platelets using hematopoietic stem cells derived from autologous bone marrow or autologous peripheral blood. When using allogeneic bone marrow or cord blood, since it is possible to use hematopoietic stem cells for which HLA almost completely coincides due to recent well-developed system of the bone marrow bank and the cord blood bank, it may become an ideal treatment method similarly.
As to in vitro platelet production technology using undifferentiated hematopoietic cells, (1) technology of inducing differentiation of undifferentiated hematopoietic cells into megakaryocytes, and (2) technology of inducing differentiation of megakaryocytes into platelets are particularly important. Regarding (1) technology of adding TPO, which is typical megakaryocyte induction factor, to the culture system has been mainly reported as basic technique. For example, a combination of TPO and several types of cytokines (Non-patent Document 3), an improvement in megakaryocyte induction/amplification efficiency by combining of TPO and various glycosaminoglycans (Non-patent Document 4) and the like, have been reported. However, a major breakthrough has not been achieved until now. Almost no reports have been made regarding (2) technology of inducing differentiation of megakaryocytes into platelets. Only some reports have been disclosed concerning the release mechanism of platelets from megakaryocytes in vivo (see Non-patent Documents 5 and 6, for example).
Specifically, in vitro platelet production technology using undifferentiated hematopoietic cells has not been completed until now, and a new technical approach has been strongly desired in this technical field. In particular, development of the technology that effectively induces differentiation into megakaryocytes and/or production of platelets from megakaryocytes is indispensable for practical applications of in vitro platelet production technology.
Moreover, this technology may be applied to platelet production technology using embryonic stem cells (ES cells) that have attracted attention as a stem cell source that may solve a deficiency in stem cells (that is pointed out when using undifferentiated hematopoietic cells from the bone marrow or cord blood), or induced pluripotent stem cells (iPS cells) that are expected to solve an ethical problem and a rejection response with ES cells (see Patent Document 1, for example). Therefore, the technical impact may be very large.